Investigation of novel antiepileptic drugs in animal seizure models is carried out as a complement to studies on the interaction of these drugs with ion channels in in vitro systems. In the present reporting period, studies were conducted examining the activity of steroid hormones in models of status epilepticus. Endogenous metabolites of certain steroid hormones (neurosteroids) can modulate the excitability of CNS neurons via direct actions on GABA/A receptors. Several structurally related metabolites of progesterone (3alpha- hydroxy pregnane-20-ones) and deoxycorticosterone (3alpha-hydroxy pregnane-21-diol-20-ones) and their 3beta-epimers were evaluated for protective activity against pilocarpine- , kainic acid- and N-methyl-D-aspartate (NMDA)-induced seizures in mice. Steroids with the 3-hydroxy group in the alpha-position and 5-H in the alpha- or beta-configurations were highly effective in protecting against pilocarpine (416 mg/kg, s.c.)-induced limbic motor seizures and status epilepticus (ED50 values, 7.018.7 mg/kg, i.p.). The corresponding epimers with the 3-hydroxy group in the beta-position were also effective but less potent (ED50 values, 33.863.5, i.p.). Although the neuroactive steroids were considerably less potent than the benzodiazepine clonazepam in protecting against pilocarpine seizures, steroids with the 5alpha,3alpha-configuration had comparable or higher protective index values (TD50 for motor impairment/ED50 for seizure protection) than clonazepam, indicating that some neuroactive steroids may have lower relative toxicity. Steroids with the 5alpha,3alpha- or 5beta,3alpha- configurations also produced a dose-dependent delay in the onset of limbic seizures induced by kainic acid (32 mg/kg, s.c.), but did not completely protect against the seizures. However, when a second dose of the steroid was administered 1 hr after the first dose, complete protection from the kainic acid-induced limbic seizures and status epilepticus was obtained. The steroids also caused a dose-dependent delay in NMDA (257 mg/kg, s.c.)-induced lethality, but did not completely protect against NMDA seizures or lethality. It was concluded that neuroactive steroids are highly effective in protecting against pilocarpine- and kainic acid-induced seizures and status epilepticus in mice, and may beuseful in treating some forms of status epilepticus in humans.